DEFLABEL

where name is a new symbolic name (see name) and doc-string
is an optional documentation string (see doc-string). This
event adds the documentation string for symbol name to the :doc data
base. By virtue of the fact that deflabel is an event, it also
marks the current history with the name. Thus, even undocumented
labels are convenient as landmarks in a proof development. For
example, you may wish to undo back through some label or compute a
theory expression (see theories) in terms of some labels.
Deflabelevents are never considered redundant.
See redundant-events.